The present invention relates to a fixing device incorporated in an image forming apparatus such as a printer, a facsimile, a copier for forming an image by using an electrophotography technology. Particularly, the invention relates to a technology of improving a sheet separator incorporated in the fixing device.
Generally, an image forming apparatus using an electrophotography technology comprises a photosensitive member having a photosensitive layer on an outer peripheral face thereof, a charger for uniformly charging the outer peripheral face of the photosensitive member, an exposer for forming an electrostatic latent image by selectively exposing the outer peripheral face uniformly charged by the charger, a developing device for forming a visible image (toner image) by providing a toner which is a developing agent to the electrostatic latent image formed by the exposer, a transferring device for transferring the toner image developed by the developing device onto a sheet-shaped recording medium (paper or the like), and a fixing device for fixing the toner image on the recording medium.
The fixing device comprises a fixing member driven to rotate (for example, a fixing roller or a fixing belt having a heat source) and a pressing member (for example, a pressing roller) rotated by being brought into press contact with the fixing member, so that the toner is melted to fix on the recording medium by heating the recording medium while passing the recording medium at a portion of bringing the fixing member and the pressing member into press contact with each other (a nip portion).
Further, a face of the recording medium formed with the toner image is liable to paste on the fixing member (also on the pressing member when images are formed on both faces of the recording medium) by adhering property of the toner. Accordingly, at least one of the fixing member and the pressing member is provided with a sheet separator for separating and guiding the recording medium, which has been passed through the nip portion, from a surface of the member.
Japanese Patent Publication No. 11-184300A (cf., descriptions [0018], [0022]–[0024], FIGS. 1 and 2) discloses a fixing device comprising: a fixing roller driven to rotate; a pressing roller rotated by being brought into press contact with the fixing roller; a separator provided at the fixing roller for separating and guiding a sheet-shaped recording medium passing a nip portion of the fixing roller and the pressing roller from a surface of the fixing roller; and a supporter for supporting the separator.
An upstream end of a guide face of the separator is brought into contact with the surface of the fixing roller, and a guide face of the supporter is bent relative to a downstream end of the guide face in a direction of moving the recording medium.
Further, the separator is constituted by a base member comprised of heat resistant resin or metal and a fluororesin layer provided at a surface of the base member. The supporter is constituted by metal.
According to the above-described fixing device, when the recording medium is guided by the guide face to a discharge roller or the like disposed at a later stage after passing the nip portion of the fixing member and the pressing member, there is a case in which the leading end of the recording medium collides on the discharge roller or the like. Therefore, there is a case in which the recording medium is temporarily bent significantly between the nip portion and the discharge roller or the like, so that a toner adhering face of the recording medium is brought into press contact with the guide face to paste thereon to bring about paper jam (clogging of recording medium).
In the above fixing device, since a downstream end of the guide face of the supporter and the downstream end of the separator are both remote from an inlet of the discharge roller pair, when the recording medium passes the nip portion of the fixing member and the pressing member and is guided by the guide faces to the discharge roller pair disposed at the later stage, the leading end of the recording medium tend to collide on the discharge roller or the like, so that paper jam (clogging of recording medium) is liable to be brought about.
In order to resolve the problem, both of the downstream end of the guide face of the supporter and the downstream end of the guide face of the separator may be arranged at a vicinity of an inlet of the discharge roller pair. However, in this case, a toner adhering face of the recording medium is pasted on the separator or the supporter to thereby pose a problem of still bringing about paper jam.
Japanese Patent Publication No. 2002-287555A discloses a fixing device wherein the separator is arranged adjacent to the fixing member and shaped so as to be identical with the shape of an outlet of the nip portion of the fixing member and the pressing member to avoid troubles due to concentration of the press contact force between the upstream end of the separator and the recording medium.
Specifically, in a case where the fixing member has an inverse crown shape (a diameter of both longitudinal end portions is larger than a diameter of a longitudinal center portion), the upstream end of the separator is convex to the upstream of the medium transporting direction. On the other hand, in a case where the fixing member has a crown shape (a diameter of both longitudinal end portions is smaller than a diameter of a longitudinal center portion), the upstream end of the separator is concaved to the downstream of the medium transporting direction.
The recording medium passing the nip portion of the fixing member and the pressing member does not necessarily pass therethrough in a state where the leading edge of the recording medium is in parallel with an axis line of the fixing member but normally passes therethrough in a state where the leading edge is skewed thereto although the amount of skewing is small.
In the latter case, the leading edge of the recording medium is actually separated from a right side or a left side thereof. In the former case, since the entire leading edge of the recording medium collides with the upstream end of the separator at the same time, large impact is temporarily acted to the separator and the recording medium.
Therefore, the fixing device poses a problem that operation of separating the recording medium is not stabilized, so that a problem that paper jam (clogging of recording medium) is liable to be brought about is posed.